JP6501984B2 - Objective optical system - Google Patents

Description

  The present invention relates to an optical system having a focusing function, and more particularly to a photographing lens such as an endoscope objective lens capable of close-up observation and other small cameras for consumer use.

  A common endoscope objective lens has a wide depth of field. In a general endoscope objective lens, the depth of field is, for example, 5 mm to 100 mm. In an endoscope equipped with such an objective lens, an image of an object is captured by an imaging device, and thereby an image of the object is provided. As an imaging device, for example, a charge coupled device (CCD) or a complementary metal oxide semiconductor (C-MOS) is used.

  In recent years, in diagnosis using an endoscope, in order to improve the accuracy of diagnosis, it is required to improve the quality of an image. In order to meet this demand, the number of pixels in imaging devices is increasing. The area of the pixels is reduced in an imaging element in which the number of pixels is increased, that is, in a high-definition imaging element.

  When one point of the object is imaged by the objective lens, a point image is formed on the image plane of the objective lens. This point image has a certain extent of spread due to the influence of diffraction. Therefore, when the area of the pixel is reduced, it is not possible to obtain an image of high quality even with a high definition imaging device unless the point image is reduced accordingly. In order to reduce the point image, it is necessary to reduce the f-number of the objective lens.

  When the size of the imaging device is the same, the number of pixels can be increased by reducing the area of the pixels. However, if the number of pixels is greatly increased, the size of the imaging device increases even if the area of the pixels is reduced. As the size of the imaging device increases, it is necessary to increase the focal length of the objective lens.

  As the f-number of the objective lens decreases or the focal length of the objective lens increases, the depth of field of the objective lens decreases. As described above, in order to obtain an image quality higher than that of the conventional image, the depth of field of the objective lens becomes narrow.

  The depth of field is a range in which a clear object image can be obtained represented by a range on the object side. When the depth of field of the objective lens becomes narrow, the range in which a clear object image can be obtained becomes narrow. In order to ensure the conventional depth of field, it is sufficient to provide the objective lens with a focusing function. Because of this, the need for an objective lens having a focusing function is increasing.

  Moreover, in the field of medical endoscopes in recent years, qualitative diagnosis of a lesion has come to be performed. In this diagnosis, it is necessary to closely observe the lesion. Because of this, in medical endoscopes, the need for an objective lens having a magnifying function (hereinafter, referred to as “magnifying endoscope objective lens”) is intensified.

  In order to magnify the lesion, it is necessary to find the lesion. Because the observation range is narrow in the magnified observation, it is not easy to find the lesion in the magnified observation. From such a thing, in a magnifying endoscope objective lens, it is necessary to be able to observe a wider range than the observation range in magnifying observation.

  In the magnified observation, the distance from the objective lens to the object position (hereinafter referred to as “object distance”) is, for example, about 1 mm to 3 mm. On the other hand, in the wide range of observation as described above (hereinafter referred to as "normal observation"), the object distance is much longer than 3 mm.

  When the optical system is configured such that the object position in normal observation and the in-focus position of the objective lens coincide, the object image in normal observation (hereinafter, referred to as “normal image”) becomes an in-focus image.

  On the other hand, the object position at the time of magnified observation is apart from the object position at the time of normal observation. Further, the object position at the time of the magnified observation is not included in the depth of field of the objective lens. Therefore, in the optical system in which the normal image is in focus, the object image (hereinafter referred to as “magnified image”) in the magnified observation does not become an in-focus image.

  In order to obtain a magnified image in focus even in magnified observation, the objective lens should have a focusing function. By the objective lens having a focusing function, it is possible to observe both the normal image and the magnified image in an in-focus state. Also from this point of view, the need for an objective lens having a focusing function is increasing.

  Patent documents 1 to 7 disclose objective lenses in which at least one lens group moves along an optical axis as a magnifying endoscope objective lens.

  Patent Document 1 discloses an objective lens composed of three lens groups. The objective lens includes, in order from the object side, a first lens group of negative refractive power, a second lens group of positive refractive power, and a third lens group of negative refractive power. During focusing, the second lens unit and the third lens unit move.

  Patent Document 2, Patent Document 5 and Patent Document 7 disclose an objective lens composed of four lens groups. The objective lens includes, in order from the object side, a first lens group of negative refractive power, a second lens group of positive refractive power, a third lens group of negative refractive power, and a fourth lens group of positive refractive power. Have. During focusing, the second lens unit and the third lens unit move.

  Patent Document 3 discloses an objective lens composed of four lens groups. The objective lens includes, in order from the object side, a first lens group of negative refractive power, a second lens group of positive refractive power, a third lens group of negative refractive power, and a fourth lens group of positive refractive power. Have. At the time of focusing, the second lens unit and the third lens unit move, or the third lens unit and the fourth lens unit move.

  Patent Document 4 discloses an objective lens composed of three lens groups. The objective lens has, in order from the object side, a first lens group of positive refractive power, a second lens group of negative refractive power, and a third lens group of positive refractive power. At the time of focusing, one lens group is moved, or a part of one lens group is moved.

  Patent Document 6 discloses an objective lens composed of three lens groups and an objective lens composed of four lens groups. The objective lens of the 3-group configuration includes, in order from the object side, a first lens group of positive refracting power, a second lens group of negative refracting power, and a third lens group of positive refracting power. During focusing, the second lens unit moves.

  The objective lens in the 4-group configuration includes, in order from the object side, a first lens group of negative refractive power, a second lens group of positive refractive power, a third lens group of negative refractive power, and a fourth lens of positive refractive power. And a group. During focusing, the second lens unit and the third lens unit move.

Patent No. 4723628 Patent No. 3722458 gazette JP, 2009-300489, A Patent No. 4834799 JP, 2015-22161, A Patent No. 5567224 gazette Patent No. 5567225 gazette

  In an endoscope (hereinafter, referred to as "magnifying endoscope") capable of magnified observation, an imaging element with a large number of pixels is used. In order to use a multi-pixel imaging device, it is necessary to form a high definition optical image. The formation of a high definition optical image can be realized by reducing the F-number of the objective lens. Therefore, even with the magnifying endoscope objective lens, it is necessary to reduce the F number.

  The objective lenses disclosed in Patent Documents 1 to 7 can not be said to be objective lenses having a sufficiently small f-number. Therefore, it is difficult to say that these objective lenses have an imaging performance corresponding to a small and high definition imaging device.

  In order to cope with a high definition imaging device, it is conceivable to reduce the F number with these objective lenses. However, even if the F-number is reduced, it is easy to predict that it is difficult to achieve the desired imaging performance. Therefore, even if the F number is reduced, the objective lens can not be said to be an objective lens corresponding to a high definition imaging device.

  In addition, in the imaging device having a large number of pixels, the miniaturization is progressing year by year. Therefore, the magnifying endoscope objective lens also needs to correspond to the miniaturization of the imaging device. However, simply reducing the size of the conventional objective lens increases the sensitivity to manufacturing errors.

  As described above, in the magnifying endoscope objective lens, the lens unit moves at the time of focusing. In focusing, the lens unit is moved and stopped. The movement is likely to cause decentering of the lens unit, and at rest the position error tends to occur. Eccentricity and position errors affect focusing accuracy. When the sensitivity to manufacturing errors is high, the influence of eccentricity and position errors on focusing accuracy becomes large.

  In particular, in the case of the magnifying endoscope objective lens, the refractive power of the lens unit moving at the time of focusing tends to be large. When the refracting power of the lens group is large, the influence on the focusing accuracy due to the eccentricity and the position error is further increased. Therefore, if the sensitivity to the manufacturing error is high, the focusing accuracy may be degraded.

  In addition, if the alignment between the optical system and the imaging device is not sufficient, the imaging performance is degraded. The sensitivity to manufacturing errors affects the accuracy of alignment between the optical system and the imaging device.

  In particular, when the F number is reduced, the alignment between the optical system and the imaging device tends to be more difficult. Therefore, if the sensitivity to the manufacturing error is high, the accuracy of alignment between the optical system and the imaging device may be reduced. As a result, the imaging performance is degraded.

  Thus, in order to achieve high focusing accuracy and high imaging performance, it is necessary to reduce the sensitivity to manufacturing errors.

  The present invention has been made in view of such problems, and it is an object of the present invention to provide an objective optical system which is not easily affected by various errors, is bright, and various aberrations are well corrected.

In order to solve the problems described above and to achieve the object, an objective optical system according to at least some embodiments of the present invention,
From the object side,
A first lens group of negative refractive power which is always stationary ,
A second lens unit of positive refractive power,
A third lens unit of negative refractive power,
A fourth lens unit having a positive refractive power which is stationary at all times, consists,
At the time of focusing from a far distance object point to a near distance object point, the second lens unit moves to the object side, and the third lens unit moves to the image side,
It is characterized in that the following conditional expressions (1), (5) and (7) are satisfied.
2 <fG2 / f <8 (1)
0.2 <(t34f−t34n) / f <0.5 (5)
−8 <fG1 / f <−2 (7)
here,
fG2 is the focal length of the second lens group,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
t34f is the distance between the third lens unit and the fourth lens unit when focusing on a far distance object point;
t34 n is the distance between the third lens unit and the fourth lens unit at the time of focusing on a near object point,
fG1 is the focal length of the first lens group,
It is.

  An objective optical system according to an embodiment of the present invention can provide an objective optical system which is not easily affected by various errors, is bright, and various aberrations are well corrected.

It is sectional drawing which shows the concrete structure of the objective optical system of this embodiment. FIG. 1 is a cross-sectional view of an objective optical system of Example 1. 5 is an aberration diagram of the objective optical system of Example 1. FIG. 5 is a cross-sectional view of an objective optical system of Example 2. FIG. 5 is an aberration diagram of the objective optical system of Example 2. FIG. FIG. 7 is a cross-sectional view of an objective optical system of Example 3. 5 is an aberration diagram of the objective optical system of Example 3. FIG. FIG. 18 is a cross-sectional view of the objective optical system of Example 4. FIG. 18 shows aberration diagrams of the objective optical system of Example 4. FIG. 18 is a cross-sectional view of the objective optical system of Example 5. FIG. 18 shows aberration diagrams of the objective optical system of Example 5.

  Hereinafter, with regard to the objective optical system according to the present embodiment, the reason and action of taking such a configuration will be described using the drawings. Note that the present invention is not limited by the following embodiments.

  The objective optical system according to the present embodiment can be used, for example, as an objective lens of an endoscope. In this case, the objective optical system according to the present embodiment can perform normal observation and magnified observation with one optical system in endoscopic observation. For this purpose, the objective optical system is composed of a plurality of lens groups, and at least one lens group of the plurality of lens groups moves on the optical axis. Thus, normal observation can be performed when focusing on a far distance object point, and magnified observation can be performed when focusing on a near distance object point. That is, the observation at the same level as the microscopic observation and the magnified observation at a higher magnification can be performed in the form of extension of the magnified observation.

The objective optical system according to this embodiment includes, in order from the object side, a first lens group of negative refractive power, a second lens group of positive refractive power, a third lens group of negative refractive power, and a third lens group of positive refractive power. The second lens group moves to the object side and the third lens group moves to the image side at the time of focusing from a far distance object point to a close distance object point, and the following conditional expression It is characterized by satisfying (1).
2 <fG2 / f <8 (1)
here,
fG2 is the focal length of the second lens group,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

  The objective optical system according to this embodiment includes, in order from the object side, a first lens group of negative refractive power, a second lens group of positive refractive power, a third lens group of negative refractive power, and a third lens group of positive refractive power. And 4 lens groups. In this way, not only can aberration variation at the time of focusing be minimized, and downsizing of the entire optical system is facilitated.

  The object point distance is different between normal observation and enlarged observation. In addition, the object point distance changes continuously between normal observation and magnified observation. In observation, it is preferable that a clear image is formed even if the object point distance changes. For that purpose, it is necessary to move at least one lens group.

  In the objective optical system according to this embodiment, focusing is performed by moving the second lens group and the third lens group. At the time of focusing from a far distance object point to a near distance object point, the second lens unit moves to the object side, and the third lens unit moves to the image side.

  By moving the second lens group and the third lens group, focusing can be performed regardless of where the object point is located between the long distance and the short distance. Normal observation can be performed when focusing on a far distance object point, and magnified observation can be performed when focusing on a near distance object point.

  The second lens group and the third lens group both have a focusing function. When the object point distance changes, the position of the image plane changes. By moving the second lens group and the third lens group, the position of the image plane does not change even if the object point distance changes.

  Also, by making the lens group used for focusing into the second lens group and the third lens group, the refractive power in the case of focusing with one lens group is dispersed to the second lens group and the third lens group It can be done. As a result, both the refractive power of the second lens group and the refractive power of the third lens group can be reduced. Therefore, even at the time of focusing, it is possible to maintain the state in which various aberrations are satisfactorily corrected.

  When focusing is performed by one lens unit, if the refractive power of the lens unit is small, the amount of movement must be increased to secure a desired focusing range. In the objective optical system according to the present embodiment, the sign of the refractive power of the second lens group is different from the sign of the refractive power of the third lens group. Therefore, even if the refractive power of each lens group is small, it is possible to secure a desired focusing range without increasing the amount of movement.

  During focusing, the second and third lens units move along the optical axis. On the other hand, the first lens unit and the fourth lens unit are always stationary. In the following description, the frame member holding the first lens group and the frame member holding the fourth lens group are used as a fixed frame, and the frame member holding the second lens group and the frame member holding the third lens group move Set as a frame.

  The fixed frame of the first lens group and the moving frame of the second lens group are different frame members. The fixed frame of the fourth lens group and the moving frame of the third lens group are other frame members.

  In the moving frame, the outer peripheral surface of the moving frame moves along the inner peripheral surface of the fixed frame, or the inner peripheral surface of the moving frame moves along the outer peripheral surface of the fixed frame. If the two surfaces are completely in contact, the moving frame can not be moved smoothly due to friction. Therefore, a clearance is provided between the two surfaces.

  In the movement of the moving frame, it is preferable that the shift amount of the center of the moving frame with respect to the center of the fixed frame be small. The smaller the amount of clearance, the smaller the amount of deviation. The amount of deviation corresponds to the amount of eccentricity of the second lens group with respect to the first lens group and the amount of eccentricity of the third lens group with respect to the fourth lens group. Therefore, the smaller the amount of clearance, the smaller the amount of eccentricity.

  However, as mentioned above, the amount of clearance can not be zero. Therefore, the decentering amount of the second lens group with respect to the first lens group and the decentering amount of the third lens group with respect to the fourth lens group can not be made zero. When decentering occurs in the optical system, imaging performance is degraded. Therefore, it is preferable to reduce the amount of eccentricity as much as possible.

  Conditional expression (1) relates to the focal length of the second lens unit. As described above, a clearance is provided between the moving frame and the fixed frame. Therefore, in the objective optical system according to the present embodiment, the decentering amount in the second lens group is larger than that in the case where the second lens group is fixed. Therefore, it is preferable to satisfy conditional expression (1).

  If the lower limit value of the conditional expression (1) is not reached, the refractive power of the second lens unit becomes too large. In this case, the amount of decentration in the second lens unit is increased, and the imaging performance is significantly degraded.

  When the upper limit value of the conditional expression (1) is exceeded, the refractive power of the second lens unit becomes too small. In this case, the amount of eccentricity in the second lens group is reduced. However, the amount of movement of the second lens group becomes large. As a result, the overall length of the optical system is increased. Therefore, it is not preferable to exceed the upper limit value of the conditional expression (1).

Instead of the conditional expression (1), it is preferable to satisfy the following conditional expression (1 ′).
2.5 <fG2 / f <8 (1 ')
It is more preferable to satisfy the following conditional expression (1 ′ ′) in place of the conditional expression (1).
3 <fG2 / f <6 (1 ”)

  By satisfying the conditional expression (1 ′) or the conditional expression (1 ′ ′), even if the second lens group is decentered with respect to the first lens group, deterioration of imaging performance and increase of the total length of the optical system It can be effectively suppressed.

  A specific configuration example of the objective optical system according to the present embodiment will be described. FIG. 1 is a cross-sectional view showing a specific configuration of an objective optical system according to the present embodiment.

  The objective optical system includes, in order from the object side, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, a third lens group G3 having negative refractive power, and And a fourth lens group G4 having a refractive power of

  The first lens group G1 includes, in order from the object side, a negative first lens L1, a positive second lens L2, and a negative third lens L3. The second lens L2 and the third lens L3 are cemented to constitute a cemented lens CL1.

  The second lens group G2 has a positive fourth lens L4.

  The third lens group G3 includes, in order from the object side, a negative fifth lens L5 and a positive sixth lens L6. The fifth lens L5 and the sixth lens L6 are cemented to constitute a cemented lens CL2.

  The fourth lens group G4 has a positive seventh lens L7, a positive eighth lens L8, and a negative ninth lens L9 in this order from the object side. The eighth lens L8 and the ninth lens L9 are cemented to constitute a cemented lens CL3.

  In addition, focusing is performed by moving the second lens group G2 and the third lens group G3. At the time of focusing from a far distance object point to a close distance object point, the second lens group G2 moves to the object side, and the third lens group G3 moves to the image side.

  The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

  A first parallel flat plate F1 is disposed between the first lens L1 and the second lens L2. The first parallel plate F1 can be disposed at any position in the objective optical system. The second parallel flat plate F2 and the third parallel flat plate F3 are disposed on the image side of the ninth lens L9. The second parallel plate F2 and the third parallel plate F3 are joined.

  The first parallel flat plate F1 is for cutting a specific wavelength, for example, laser light of YAG laser (light of 1060 nm wavelength), laser light of semiconductor laser (light of wavelength 810 nm), or light of wavelength of near infrared region Filter.

  The second parallel flat plate F2 and the third parallel flat plate F3 are cover glasses of the imaging device. An image sensor (not shown) is disposed on the image side of the third parallel plate F3. An image side surface of the third parallel flat plate F3 is an image plane (imaging plane) I. The imaging surface of the imaging element coincides with the image side surface of the third parallel flat plate F3.

The objective optical system according to the present embodiment preferably satisfies the following conditional expression (2).
-8 <fG3 / f <-2 (2)
here,
fG3 is the focal length of the third lens unit,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

  Conditional expression (2) relates to the focal length of the third lens unit. As described above, a clearance is provided between the moving frame and the fixed frame. Therefore, in the objective optical system according to the present embodiment, the decentering amount in the third lens group is larger than that in the case where the third lens group is fixed. Therefore, it is preferable to satisfy conditional expression (2).

  When the lower limit value of the conditional expression (2) is not reached, the refractive power of the third lens unit becomes too small. In this case, the amount of eccentricity in the third lens group is reduced. However, the amount of movement of the third lens group is increased. As a result, the overall length of the optical system is increased. Therefore, it is not preferable to fall below the lower limit value of the conditional expression (2).

  If the upper limit value of the conditional expression (2) is exceeded, the refractive power of the second lens unit becomes too large. In this case, the amount of decentration in the third lens unit is increased, so that the imaging performance is significantly degraded.

The objective optical system according to the present embodiment preferably satisfies the following conditional expression (3).
0.5 <(t12f−t12n) / (t34f−t34n) <4 (3)
here,
t12f is the distance between the first lens unit and the second lens unit when focusing on a far distance object point;
t12 n is the distance between the first lens unit and the second lens unit at the time of focusing on a near object point;
t34f is the distance between the third lens unit and the fourth lens unit when focusing on a far distance object point;
t34 n is the distance between the third lens unit and the fourth lens unit at the time of focusing on a near object point,
It is.

  Conditional expression (3) relates to the amount of movement of the second lens unit and the amount of movement of the third lens unit.

  Below the lower limit value of the conditional expression (3), the distance between the first lens group and the second lens group becomes too short. Therefore, it becomes difficult to secure a space necessary for moving the second lens unit. If the upper limit value of the conditional expression (3) is exceeded, the distance between the third lens unit and the fourth lens unit becomes too short. Therefore, it becomes difficult to secure a space necessary for moving the third lens unit.

  In the objective optical system according to the present embodiment, among the focusable distances, the distance closest to the optical system can be set to, for example, about 2 mm. For that purpose, it is preferable to satisfy conditional expression (3).

  When the conditional expression (3) is not satisfied, it is difficult to secure the moving amount of the second lens unit and the moving amount of the third lens unit. Therefore, it becomes impossible to focus on an object point located at a short distance, for example, an object point located at a distance of about 2 mm from the optical system. As a result, observation at a sufficiently large magnification becomes difficult.

  In addition, when a space necessary for movement can not be secured, the movement amount of the second lens unit must be reduced or the movement amount of the third lens unit must be reduced so as to be contained in a narrow space.

  In order to reduce the amount of movement of the lens group, it is necessary to increase the refractive power of the lens group. However, when the refractive power of the lens unit is increased, the image surface position sensitivity in the second lens unit is increased, or the image surface position sensitivity in the third lens unit is increased.

  As a result, a defect due to a manufacturing error, for example, a defect such as an increase in image surface position shift due to a displacement of the second lens unit or a defect such as an increase in image surface displacement due to a displacement of the third lens unit tends to occur. . The image plane position sensitivity is the ratio of the image plane position shift to the lens unit position shift.

The objective optical system according to the present embodiment preferably satisfies the following conditional expression (4).
0.2 <(t12f−t12n) / f <1.2 (4)
here,
t12f is the distance between the first lens unit and the second lens unit when focusing on a far distance object point;
t12 n is the distance between the first lens unit and the second lens unit at the time of focusing on a near object point;
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

  Below the lower limit value of the conditional expression (4), it becomes difficult to secure a space necessary for the movement of the second lens group, as with the lower limit value of the conditional expression (3). If the upper limit value of the conditional expression (4) is exceeded, the distance between the first lens unit and the second lens unit becomes too large. In this case, a space necessary for moving the second lens unit can be secured. However, this leads to an increase in the size of the optical system.

The objective optical system according to the present embodiment preferably satisfies the following conditional expression (5).
0.2 <(t34f−t34n) / f <0.5 (5)
here,
t34f is the distance between the third lens unit and the fourth lens unit when focusing on a far distance object point;
t34 n is the distance between the third lens unit and the fourth lens unit at the time of focusing on a near object point,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

  Below the lower limit value of the conditional expression (5), it becomes difficult to secure a space necessary for the movement of the third lens group, as in the case of exceeding the upper limit value of the conditional expression (3). If the upper limit value of the conditional expression (5) is exceeded, the distance between the third lens unit and the fourth lens unit becomes too large. In this case, a space necessary for moving the third lens group can be secured. However, this leads to an increase in the size of the optical system.

The objective optical system according to the present embodiment preferably satisfies the following conditional expression (6).
1 <fG4 / f <3.5 (6)
here,
fG4 is the focal length of the fourth lens unit,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

  Conditional expression (6) relates to correction of curvature of field.

  When the lower limit value of the conditional expression (6) is not reached, the image surface is inclined to under. When the upper limit of conditional expression (6) is exceeded, the image surface is inclined to over. In this case, either the central part or the peripheral part of the image is out of focus. Therefore, it is not preferable to fall below the lower limit value of conditional expression (6) or to exceed the upper limit value.

The objective optical system according to the present embodiment preferably satisfies the following conditional expression (7).
−8 <fG1 / f <−2 (7)
here,
fG1 is the focal length of the first lens group,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

  Conditional expression (7) is a conditional expression regarding correction of lateral chromatic aberration.

  Below the lower limit value of the conditional expression (7), both lateral chromatic aberration at the C-line and lateral chromatic aberration at the F-line become overcorrected. Therefore, it is not preferable to fall below the lower limit value of the conditional expression (7).

  If the upper limit value of the conditional expression (7) is exceeded, the balance between the axial chromatic aberration at the C-line and the axial chromatic aberration at the F-line is broken. In addition, lateral chromatic aberration is undercorrected. In this case, in the periphery of the image, a drop in contrast accompanied by color bleeding is caused. Therefore, it is not preferable to exceed the upper limit value of the conditional expression (7).

The objective optical system according to the present embodiment preferably satisfies the following conditional expression (8).
−1.8 <fG2 / fG3 <−1 (8)
here,
fG2 is the focal length of the second lens group,
fG3 is the focal length of the third lens unit,
It is.

  The conditional expression (8) is a conditional expression for making both the refractive power of the second lens group and the refractive power of the third lens group appropriate. In other words, the conditional expression (8) suppresses the fluctuation of the image plane position at the time of focusing, and contributes to the downsizing of the optical system.

  If the lower limit value of the conditional expression (8) is not reached, the refractive power of the second lens unit becomes too small. In this case, the amount of movement of the second lens unit becomes too large. This leads to an increase in the size of the optical system.

  If the upper limit value of the conditional expression (8) is exceeded, the refractive power of the third lens group becomes too large. In this case, fluctuation of curvature of field due to focusing becomes large. Therefore, there is a significant difference between the image plane position in the normal observation and the image plane position in the close observation. Therefore, it is not preferable to exceed the upper limit value of the conditional expression (8).

In the objective optical system according to the present embodiment, the fourth lens group at least includes a first sub lens group and a second sub lens group in order from the object side, and the first sub lens group includes a positive lens. It is preferable that the second sub lens unit includes a cemented lens including a positive lens and a negative lens, and the following conditional expression (9) is satisfied.
0.45 <fG4SUB1 / fG4SUB2 <1.15 (9)
here,
fG4SUB1 is a focal length of the first sub lens unit,
fG4SUB2 is a focal length of the second sub lens unit,
It is.

  Conditional expression (9) relates to the ratio of the refractive powers of the two sub lens units constituting the fourth lens unit. By making the refracting powers of the two sub lens units be appropriate refracting powers, it is possible to minimize the variation that is effective in correcting the field curvature.

  If the lower limit value of the conditional expression (9) is exceeded, the curvature of field is excessively under. If the upper limit value of the conditional expression (9) is exceeded, the curvature of field becomes excessive. Therefore, it is not preferable to fall below the lower limit value of conditional expression (9) or to exceed the upper limit value.

In the objective optical system according to the present embodiment, the first lens group has an object side lens and a sub lens group of positive refracting power, the object side lens is located most on the object side, and the sub lens group is It is preferable that the zoom lens be positioned on the image side of the object side lens and satisfy the following conditional expression (10).
−8 <fG1Lo / fG1SUB <−3.5 (10)
here,
fG1Lo is the focal length of the object-side lens,
fG1SUB is the focal length of the sub lens group,
It is.

  The conditional expression (10) relates to the correction of the change of the curvature of field due to the change of the object point distance.

  When the lower limit value of the conditional expression (10) is not reached, the curvature of field at the far object point becomes largely under. If the upper limit value of the conditional expression (10) is exceeded, the curvature of field at the near object point becomes largely under. Therefore, it is not preferable to fall below the lower limit value of conditional expression (10) or to exceed the upper limit value.

In the objective optical system according to this embodiment, it is preferable that the first lens group has a cemented lens including a first positive lens and a first negative lens, and the following conditional expression (11) is satisfied.
−1.6 <fG1Cp / fG1Cn <−0.4 (11)
fG1Cp is the focal length of the first positive lens,
fG1Cn is the focal length of the first negative lens,
It is.

  Conditional expression (11) relates to correction of longitudinal chromatic aberration and correction of lateral chromatic aberration. In the first lens group, a change in chromatic aberration occurs due to a change in object point distance. By satisfying the conditional expression (11), it is possible to suppress the fluctuation of this chromatic aberration to a small extent.

  Below the lower limit value of the conditional expression (11), lateral chromatic aberration of magnification at the time of near object point focusing becomes large. Therefore, it is not preferable to fall below the lower limit value of the conditional expression (11).

  If the upper limit value of the conditional expression (11) is exceeded, the fluctuation of the axial chromatic aberration with respect to the change of the object point distance becomes large. In particular, axial chromatic aberration at the time of long distance object point focusing becomes large. As described above, the fluctuation of the axial chromatic aberration and the increase of the axial chromatic aberration cause the reduction of the contrast. Therefore, it is not preferable to exceed the upper limit value of the conditional expression (11).

The objective optical system according to the present embodiment preferably has a brightness stop and satisfies the following conditional expression (12).
0.3 <fGF / fGR <0.75 (12)
here,
fGF is the focal length of the front group at long distance object point focusing,
fGR is the focal length of the rear group at long distance object point focusing,
The front group is a lens group composed of all lens groups located on the object side of the aperture stop,
The rear group is a lens group composed of all lens groups located on the image side of the aperture stop,
It is.

  The objective optical system according to the present embodiment can be divided into a front group and a rear group with the brightness stop as a boundary. The front group is a lens group configured by all the lens groups located on the object side of the aperture stop. The rear group is a lens group configured by all the lens groups positioned on the image side of the brightness stop.

  The ratio of the refractive power of the front group to the refractive power of the rear group is preferably within the range of the conditional expression (12). If the ratio of the refractive power of the front group to the refractive power of the rear group is within the range of the conditional expression (12), it is possible to correct the field curvature well.

  When the lower limit value of the conditional expression (12) is not reached, the image plane is greatly inclined to over. If the upper limit value of the conditional expression (12) is exceeded, the image plane is greatly inclined to under. Therefore, it is not preferable to fall below the lower limit value of conditional expression (12) or to exceed the upper limit value.

It is preferable that the following conditional expression (12 ′) be satisfied instead of the conditional expression (12).
0.38 <fGF / fGR <0.52 (12 ')

  If the refractive power of the front group and the refractive power of the rear group are within the range of the conditional expression (12 '), it is possible to correct the curvature of field better.

The objective optical system according to the present embodiment preferably satisfies the following conditional expression (13).
2.5 <Fno <5 (13)
here,
Fno is the f-number when focusing on a far distance object point,
It is.

  Conditional expression (13) relates to the F number of the objective optical system according to the present embodiment.

  If the lower limit value of the conditional expression (13) is exceeded, a bright optical system can be realized, but the depth of field becomes shallow. Therefore, it is not preferable to fall below the lower limit value of the conditional expression (13).

  In high definition imaging devices, the spatial frequency that can be reproduced is high. Therefore, the objective optical system also needs to have high imaging performance at the spatial frequency corresponding to the high definition imaging device. To that end, the point image formed by the objective optical system must be reduced.

  The size of the point image is affected by diffraction. In order to reduce the point image, it is necessary to reduce the f-number of the objective optical system.

  If the upper limit value of the conditional expression (13) is exceeded, the F number of the objective optical system can not be made sufficiently small. In this case, the point image can not be reduced due to the influence of diffraction. That is, it is not possible to enhance the imaging performance at the spatial frequency corresponding to the high definition imaging device.

  In the objective optical system according to the present embodiment, a cemented lens may be disposed on the object side of the aperture stop and in the vicinity of the aperture stop.

  Correction of axial chromatic aberration is important to have high imaging performance at spatial frequency corresponding to a high definition imaging device. By arranging the cemented lens on the object side of the aperture stop and next to the aperture stop, sufficient correction of axial chromatic aberration becomes possible.

  The cemented lens may be disposed near the aperture stop. Therefore, the place where the cemented lens is disposed may not be adjacent to the aperture stop. For example, a cemented lens may be disposed with a single lens interposed therebetween. Even in this case, sufficient correction of axial chromatic aberration is possible.

  The objective optical system according to the present embodiment can also be used for optical devices other than endoscopes.

  For example, the objective optical system according to the present embodiment can be used for an imaging optical system of a digital camera. When shooting with a digital camera, macro shooting that exceeds equal magnification may be performed. In such a case, the macro converter lens is often mounted because the amount of extension of the lens may be large. However, by using the objective optical system of the present embodiment as an imaging optical system, it is possible to perform macro photographing of a higher magnification than ever without mounting a macro converter lens.

  Also, in general, in the macro lens, the first lens group is extended to the object side, and focusing is performed by floating of a plurality of lens groups. On the other hand, if the objective optical system of the present embodiment is used, macro photography with the inner focus becomes possible. Therefore, it is advantageous when photographing after determining the working distance.

  Furthermore, the objective optical system according to the present embodiment can also be used in an imaging optical system of a portable device such as a camera of a mobile phone. By doing this, macro photography can be enjoyed easily.

  Below, the Example of an objective optical system is described in detail based on drawing. The present invention is not limited by this embodiment.

  The lens sectional view of each example will be described. (A) is a cross-sectional view in a normal observation state, (b) is a cross-sectional view in a magnified observation state.

  Aberration diagrams of each example will be described. (A), (b), (c) and (d) are aberration diagrams in the normal observation state, respectively. (A) shows spherical aberration (SA), (b) shows astigmatism (AS), (c) shows distortion (DT), and (d) shows lateral chromatic aberration (CC).

  (E), (f), (g) and (h) are aberration diagrams in the magnified observation state, respectively. (E) indicates spherical aberration (SA), (f) indicates astigmatism (AS), (g) indicates distortion (DT), and (h) indicates lateral chromatic aberration (CC).

  In each aberration diagram, the horizontal axis represents the amount of aberration. For spherical aberration, astigmatism and magnification aberration, the unit of the amount of aberration is mm. In addition, with respect to distortion, the unit of the amount of aberration is%. Further, ω is a half angle of view, the unit is ° (degree), and FNO is an F number. The unit of the wavelength of the aberration curve is nm.

Example 1
An objective optical system according to Example 1 will be described. The objective optical system of Example 1 includes, in order from the object side, a first lens group G1 of negative refractive power, a second lens group G2 of positive refractive power, a third lens group G3 of negative refractive power, and positive refractive power. And the fourth lens group G4.

  The first lens group G1 has a plano-concave negative lens L1 having a flat surface on the object side, a positive meniscus lens L2 having a convex surface facing the image side, and a biconcave negative lens L3. Here, a cemented lens CL1 is formed by the positive meniscus lens L2 and the biconcave negative lens L3.

  The second lens group G2 has a biconvex positive lens L4.

  The third lens group G3 has a biconcave negative lens L5 and a positive meniscus lens L6 with a convex surface facing the object side. Here, the cemented lens CL2 is formed of the biconcave negative lens L5 and the positive meniscus lens L6.

  The fourth lens group G4 has a biconvex positive lens L7, a biconvex positive lens L8, and a negative meniscus lens L9 having a convex surface directed to the image side. Here, the cemented lens CL3 is formed by the biconvex positive lens L8 and the negative meniscus lens L9.

  The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

  During focusing, the second lens group G2 and the third lens group G3 move. When focusing on a near distance object point after focusing on a far distance object point, the second lens group G2 moves to the object side, and the third lens group G3 moves to the image side.

  A parallel plate F1 is disposed on the image side of the plano-concave negative lens L1. A parallel flat plate F2 and a parallel flat plate F3 are disposed on the image side of the fourth lens group G4.

  The parallel plate F1 is a filter for cutting a specific wavelength, for example, laser light of YAG laser (light of 1060 nm wavelength), laser light of semiconductor laser (light of wavelength 810 nm), or light of wavelength of near infrared region. is there. The plane parallel plate F2 and the plane parallel plate F3 are cover glasses for the imaging device.

(Example 2)
An objective optical system according to the second embodiment will be described. The objective optical system of Example 2 includes, in order from the object side, a first lens group G1 of negative refractive power, a second lens group G2 of positive refractive power, a third lens group G3 of negative refractive power, and positive refractive power. And the fourth lens group G4.

  The first lens group G1 has a plano-concave negative lens L1 having a flat surface on the object side, a positive meniscus lens L2 having a convex surface on the image side, and a negative meniscus lens L3 having a convex surface on the image side. Here, the cemented lens CL1 is formed by the positive meniscus lens L2 and the negative meniscus lens L3.

  The second lens group G2 has a biconvex positive lens L4.

  The third lens group G3 has a biconcave negative lens L5 and a positive meniscus lens L6 with a convex surface facing the object side. Here, the cemented lens CL2 is formed of the biconcave negative lens L5 and the positive meniscus lens L6.

  The fourth lens group G4 has a biconvex positive lens L7, a biconvex positive lens L8, and a negative meniscus lens L9 having a convex surface directed to the image side. Here, the cemented lens CL3 is formed by the biconvex positive lens L8 and the negative meniscus lens L9.

  The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

  During focusing, the second lens group G2 and the third lens group G3 move. When focusing on a near distance object point after focusing on a far distance object point, the second lens group G2 moves to the object side, and the third lens group G3 moves to the image side.

  A parallel plate F1 is disposed on the image side of the plano-concave negative lens L1. A parallel flat plate F2 and a parallel flat plate F3 are disposed on the image side of the fourth lens group G4.

  The parallel plate F1 is a filter for cutting a specific wavelength, for example, laser light of YAG laser (light of 1060 nm wavelength), laser light of semiconductor laser (light of wavelength 810 nm), or light of wavelength of near infrared region. is there. The plane parallel plate F2 and the plane parallel plate F3 are cover glasses for the imaging device.

(Example 3)
An objective optical system according to Example 3 will be described. The objective optical system of Example 3 includes, in order from the object side, a first lens group G1 of negative refractive power, a second lens group G2 of positive refractive power, a third lens group G3 of negative refractive power, and positive refractive power. And the fourth lens group G4.

  The first lens group G1 has a plano-concave negative lens L1 having a flat surface on the object side, a positive meniscus lens L2 with a convex surface facing the image side, a biconcave negative lens L3, and a biconvex positive lens L4. Here, the cemented lens CL1 is formed by the biconcave negative lens L3 and the biconvex positive lens L4.

  The second lens group G2 has a biconvex positive lens L5.

  The third lens group G3 has a biconcave negative lens L6 and a positive meniscus lens L7 with a convex surface facing the object side. Here, the cemented lens CL2 is formed of the biconcave negative lens L6 and the positive meniscus lens L7.

  The fourth lens group G4 has a biconvex positive lens L8, a biconvex positive lens L9, and a biconcave negative lens L10. Here, the cemented lens CL3 is formed by the biconvex positive lens L9 and the biconcave negative lens L10.

  The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

  During focusing, the second lens group G2 and the third lens group G3 move. When focusing on a near distance object point after focusing on a far distance object point, the second lens group G2 moves to the object side, and the third lens group G3 moves to the image side.

  The parallel flat plate F1, the parallel flat plate F2, and the parallel flat plate F3 are disposed on the image side of the fourth lens group G4.

  The parallel plate F1 is a filter for cutting a specific wavelength, for example, laser light of YAG laser (light of 1060 nm wavelength), laser light of semiconductor laser (light of wavelength 810 nm), or light of wavelength of near infrared region. is there. The plane parallel plate F2 and the plane parallel plate F3 are cover glasses for the imaging device.

(Example 4)
An objective optical system according to Example 4 will be described. The objective optical system of Example 4 includes, in order from the object side, a first lens group G1 of negative refractive power, a second lens group G2 of positive refractive power, a third lens group G3 of negative refractive power, and positive refractive power. And the fourth lens group G4.

  The first lens group G1 has a plano-concave negative lens L1 having a flat object side, a biconvex positive lens L2, a biconcave negative lens L3, and a biconvex positive lens L4. Here, the cemented lens CL1 is formed by the biconcave negative lens L3 and the biconvex positive lens L4.

  The second lens group G2 has a biconvex positive lens L5.

  The third lens group G3 has a biconcave negative lens L6.

  The fourth lens group G4 has a biconvex positive lens L7, a biconvex positive lens L8, and a negative meniscus lens L9 having a convex surface directed to the image side. Here, the cemented lens CL3 is formed by the biconvex positive lens L8 and the negative meniscus lens L9.

  The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

  During focusing, the second lens group G2 and the third lens group G3 move. When focusing on a near distance object point after focusing on a far distance object point, the second lens group G2 moves to the object side, and the third lens group G3 moves to the image side.

  The parallel flat plate F1, the parallel flat plate F2, and the parallel flat plate F3 are disposed on the image side of the fourth lens group G4.

  The parallel plate F1 is a filter for cutting a specific wavelength, for example, laser light of YAG laser (light of 1060 nm wavelength), laser light of semiconductor laser (light of wavelength 810 nm), or light of wavelength of near infrared region. is there. The plane parallel plate F2 and the plane parallel plate F3 are cover glasses for the imaging device.

(Example 5)
An objective optical system according to Example 5 will be described. The objective optical system of Example 5 includes, in order from the object side, a first lens group G1 of negative refractive power, a second lens group G2 of positive refractive power, a third lens group G3 of negative refractive power, and positive refractive power. And the fourth lens group G4.

  The first lens group G1 has a plano-concave negative lens L1 having a flat surface on the object side, a positive meniscus lens L2 with a convex surface facing the image side, a biconcave negative lens L3, and a biconvex positive lens L4. Here, the cemented lens CL1 is formed by the biconcave negative lens L3 and the biconvex positive lens L4.

  The second lens group G2 has a biconvex positive lens L5.

  The third lens group G3 has a biconcave negative lens L6.

  The fourth lens group G4 has a biconvex positive lens L7, a biconvex positive lens L8, and a negative meniscus lens L9 having a convex surface directed to the image side. Here, the cemented lens CL3 is formed by the biconvex positive lens L8 and the negative meniscus lens L9.

  The aperture stop S is disposed between the second lens group G2 and the third lens group G3.

  During focusing, the second lens group G2 and the third lens group G3 move. When focusing on a near distance object point after focusing on a far distance object point, the second lens group G2 moves to the object side, and the third lens group G3 moves to the image side.

  The parallel flat plate F1, the parallel flat plate F2, and the parallel flat plate F3 are disposed on the image side of the fourth lens group G4.

  The parallel plate F1 is a filter for cutting a specific wavelength, for example, laser light of YAG laser (light of 1060 nm wavelength), laser light of semiconductor laser (light of wavelength 810 nm), or light of wavelength of near infrared region. is there. The plane parallel plate F2 and the plane parallel plate F3 are cover glasses for the imaging device.

  Below, numerical data of each of the above examples are shown. In the surface data, r is the radius of curvature of each lens surface, d is the distance between each lens surface, ne is the refractive index of the e-line of each lens, and d d is the Abbe number of each lens.

  In various data, f is the focal length at the e-line, Fno is the F number, ω is the half angle of view, IH is the image height, and OBJ is the object distance. In the close observation state, magnified observation (normal observation) can be performed. The aperture is the aperture stop.

Numerical embodiment 1
Unit mm

Plane data Plane number rd ne dd
1 0.3 0.383 1.88815 40.76
2 1.4062 1.093
3 0.6 0.638 1.49557 75.00
4 0.6 0.606
5 -215.6063 2.049 1.77621 49.60
6 -1.7919 0.447 1.59911 39.24
7 59.7123 Variable
8 2.7313 0.717 1.59667 35.31
9-7.2342 Variable
10 (stop) ∞ variable
11 -4.1724 0.441 1.70442 30.13
12 2.7263 0.478 1.48915 70.23
13 4.4792 Variable
14 5.7382 0.638 1.77621 49.60
15 -7.8439 0.032
16 5.5805 1.289 1.69979 55.53
17-2.1954 0.4785 1.93429 18.90
18 -4.244 1.203
19 1. 1.45 1.51825 64.14
20 0.5 0.55 1.88815 40.76
21 (imaging plane) ∞

Various data
Normal observation state Close observation state f 1.038 1.249
Fno 4.2 4.728
OBJ 26.3 4.35
d7 0.4436 0.1823
d9 0.1905 0.4518
d10 0.4492 0.7865
d13 0.6767 0.3394
IH 1.0
2ω 160.5

Numerical embodiment 2
Unit mm

Plane data Plane number rd ne dd
1 0.3 0.383 1.88815 40.76
2 1.3878 1.099
3 0.6 0.638 1.49557 75.00
4 0.6 0.6
5 -6.5257 1.542 1.77621 49.60
6-2.003 0.45 1.59911 39.24
7-5.8083 Variable
8 3.3916 0.718 1.59667 35.31
9 -8.4609 Variable
10 (stop) ∞ variable
11 -7.1747 0.44 1.7044 30.13
12 2.3594 0.473 1.48915 70.23
13 4.8045 Variable
14 4.8345 0.718 1.77621 49.60
15 -7.9744 0.032
16 5.5772 1.358 1.69979 55.53
17 -2.4477 0.478 1.93429 18.90
18-6.2241 0.964
19 1. 1.45 1.51825 64.14
20 0.5 0.56 1.51825 64.14
21 (imaging plane) ∞

Various data
Normal observation state Close observation state f 1.015 1.184
Fno 3.725 4.042
OBJ 26.2 5.25
d7 0.5128 0.1576
d9 0.1868 0.5420
d10 0.3714 0.6516
d13 0.6258 0.3456
IH 1.0
2ω 158.3

Numerical embodiment 3
Unit mm

Plane data Plane number rd ne dd
1 0.3 0.383 1.88815 40.76
2 1.317 1.176
3-7.4132 0.797 1.51825 64.14
4-2.3089 0.374
5 -1.9522 0.542 1.72733 29.23
6 11.8496 0.723 1.77621 49.60
7-2.5443 variable
8 2.981 0.718 1.59667 35.31
9-24.2805 Variable
10 (stop) ∞ variable
11-5.8402 0.44 1.7044 30.13
12 25.1198 0.4585 1.48915 70.23
13 2.9224 Variable
14 3.8937 0.766 1.77621 49.60
15-17.1962 0.032
16 2.5653 1.358 1.69979 55.53
17 -2.7452 0.4785 1.97189 17.47
18 88.9617 0.382
19 0.45 0.45 1.51825 64.14
20 0.6 0.6586
21 0.5 0.55 1.56606 60.67
22 (imaging plane) ∞

Various data
Normal observation state Close observation state f 1.049 1.334
Fno 3.615 3.956
OBJ 26.35 2.55
d7 0.9606 0.0697
d9 0.1884 1.0793
d10 0.3962 0.7026
d13 0.6377 0.3313
IH 1.0
2ω 160.2

Numerical embodiment 4
Unit mm

Surface data
Face number rd ne dd
1 0.3 0.383 1.88815 40.76
2 1.3137 1.054
3 153.3896 0.797 1.51825 64.14
4-3.5841 0.38
5-2.2715 0.542 1.72733 29.23
6 7.377 0.752 1.77621 49.60
7 -2.6647 variable
8 2.832 0.718 1.59667 35.31
9-19.9623 Variable
10 (stop) ∞ variable
11 -6.107 0.903 1.70442 30.13
12 4.5555 Variable
13 3.8753 0.766 1.77621 49.60
14-46.7115 0.032
15 2.6535 1.325 1.69979 55.53
16-2.8117 0.478 1.97189 17.47
17-38. 5243 0.122
18 0.3 0.32 1.51825 64.14
19 ∞ 0.5
20 0.4 0.48 1.51825 64.14
21 0.4 0.4 1.51825 64.14
22 (imaging plane) ∞

Various data
Normal observation state Close observation state f 1.075 1.318
Fno 3.454 3.752
OBJ 26.3 2.66
d7 0.772 0.0706
d9 0.1884 0.8898
d10 0.4165 0.7444
d13 0.6307 0.3028
IH 1.0
2ω 159.7

Numerical embodiment 5
Unit mm

Plane data Plane number rd ne dd
1 0.3 0.383 1.88815 40.76
2 1.3455 1.162
3 -5.3608 0.797 1.51825 64.14
4-2.5651 0.375
5-2.1661 0.542 1.72733 29.23
6 3.6031 0.871 1.77621 49.60
7 -2.6936 Variable
8 2.9415 0.718 1.59667 35.31
9-27.8209 Variable
10 (stop) ∞ variable
11-2.8 799 0.901 1.48915 70.23
12 2.9507 Variable
13 3.5161 0.766 1.77621 49.60
14-10.2894 0.031
15 3.4931 1.447 1.69979 55.53
16-2.0392 0.478 1.97189 17.47
17-13.4569 0.082
18 0.45 0.45 1.51825 64.14
19 0.4 0.48
20 ∞ 1 1.51825 64.14
21 0.5 0.55 1.56606 60.67
22 (imaging plane) ∞

Various data
Normal observation state Close observation state f 1.038 1.339
Fno 3.643 4.002
OBJ 28.5 2.65
d7 0.9783 0.1185
d9 0.2371 1.0969
d10 0.41 0.6936
d12 0.6329 0.3493
IH 1.0
2ω 152.8

Next, values of conditional expressions (1) to (13) in each example will be listed.

Conditional Expression Example 1 Example 2 Example 3
(1) fG2 / f 3.290 4.091 4.284
(2) fG3 / f -2.576-3.260-3.088
(3) (t12f-t12n) / (t34f-t34n) 0.775 1.268 2.908
(4) (t12f-t12n) / f 0.252 0.350 0.849
(5) (t34f-t34n) / f 0.325 0.276 0.292
(6) fG4 / f 2.350 2.521 2.256
(7) fG1 / f-2.622-3.320-4.937
(8) fG2 / fG3 -1.278 -1.255 -1.387
(9) fG4SUB1 / fG4SUB2 0.987 0.687 0.734
(10) fG1Lo / fG1SUB -7.358-6.880-4.656
(11) fG1Cp / fG1Cn -0.800 -0.607 -0.822
(12) fGF / fGR 0.464 0.461 0.416
(13) Fno 4.200 3.725 3.615

Conditional Expression Example 4 Example 5
(1) fG2 / f 3.913 4.334
(2) fG3 / f -3.329-2.733
(3) (t12f-t12n) / (t34f-t34n) 2.139 3.032
(4) (t12f-t12n) / f 0.652 0.829
(5) (t34f-t34n) / f 0.305 0.273
(6) fG4 / f 2.254 2.236
(7) fG1 / f-4.915-4.972
(8) fG2 / fG3 -1.175 -1.586
(9) fG4SUB1 / fG4SUB2 0.955 0.512
(10) fG1Lo / fG1SUB -4.244 -5.008
(11) fG1Cp / fG1Cn -0.895 -0.847
(12) fGF / fGR 0.440 0.421
(13) Fno 3.454 3.643

  As mentioned above, although various embodiments of the present invention were described, the present invention is not restricted only to these embodiments, and is an embodiment which constituted combining the composition of these embodiments suitably in the range which does not deviate from the meaning. The form is also within the scope of the present invention.

(Supplementary note)
The invention of the following configuration is derived from these examples.
(Appendix 1)
From the object side,
A first lens unit of negative refractive power,
A second lens unit of positive refractive power,
A third lens unit of negative refractive power,
And a fourth lens unit having a positive refractive power,
At the time of focusing from a far distance object point to a near distance object point, the second lens unit moves to the object side, and the third lens unit moves to the image side,
An objective optical system characterized by satisfying the following conditional expression (1).
2 <fG2 / f <8 (1)
here,
fG2 is the focal length of the second lens group,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

(Appendix 2)
The objective optical system according to item 1, wherein the following conditional expression (2) is satisfied.
-8 <fG3 / f <-2 (2)
here,
fG3 is the focal length of the third lens unit,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

(Appendix 3)
The objective optical system according to item 1 or 2, wherein the following conditional expression (3) is satisfied.
0.5 <(t12f−t12n) / (t34f−t34n) <4 (3)
here,
t12f is the distance between the first lens unit and the second lens unit when focusing on a far distance object point;
t12 n is the distance between the first lens unit and the second lens unit at the time of focusing on a near object point;
t34f is the distance between the third lens unit and the fourth lens unit when focusing on a far distance object point;
t34 n is the distance between the third lens unit and the fourth lens unit at the time of focusing on a near object point,
It is.

(Appendix 4)
The objective optical system according to any one of Appendices 1 to 3, wherein the following conditional expression (4) is satisfied.
0.2 <(t12f−t12n) / f <1.2 (4)
here,
t12f is the distance between the first lens unit and the second lens unit when focusing on a far distance object point;
t12 n is the distance between the first lens unit and the second lens unit at the time of focusing on a near object point;
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

(Appendix 5)
The objective optical system according to any one of Appendices 1 to 3, wherein the following conditional expression (5) is satisfied.
0.2 <(t34f−t34n) / f <0.5 (5)
here,
t34f is the distance between the third lens unit and the fourth lens unit when focusing on a far distance object point;
t34 n is the distance between the third lens unit and the fourth lens unit at the time of focusing on a near object point,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

(Appendix 6)
The objective optical system according to any one of Appendices 1 to 3, wherein the following conditional expression (6) is satisfied.
1 <fG4 / f <3.5 (6)
here,
fG4 is the focal length of the fourth lens unit,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

(Appendix 7)
The objective optical system according to any one of Additional Items 1 to 3, characterized by satisfying the following conditional expression (7).
−8 <fG1 / f <−2 (7)
here,
fG1 is the focal length of the first lens group,
f is the focal length of the entire objective optical system when focusing on a far distance object point,
It is.

(Appendix 8)
The objective optical system according to any one of Additional Items 1 to 3, characterized by satisfying the following conditional expression (8).
−1.8 <fG2 / fG3 <−1 (8)
here,
fG2 is the focal length of the second lens group,
fG3 is the focal length of the third lens unit,
It is.

(Appendix 9)
The fourth lens group at least includes a first sub lens group and a second sub lens group in order from the object side,
The first sub lens group has a positive lens,
The second sub lens unit has a cemented lens composed of a positive lens and a negative lens,
The objective optical system according to any one of appendices 1 to 3, characterized by satisfying the following conditional expression (9).
0.45 <fG4SUB1 / fG4SUB2 <1.15 (9)
here,
fG4SUB1 is a focal length of the first sub lens unit,
fG4SUB2 is a focal length of the second sub lens unit,
It is.

(Appendix 10)
The first lens group has an object side lens and a sub lens group of positive refracting power,
The object side lens is located closest to the object side,
The sub lens unit is located on the image side of the object side lens,
The objective optical system according to any one of appendices 1 to 3, characterized by satisfying the following conditional expression (10).
−8 <fG1Lo / fG1SUB <−3.5 (10)
here,
fG1Lo is the focal length of the object-side lens,
fG1SUB is the focal length of the sub lens group,
It is.

(Appendix 11)
The first lens group has a cemented lens consisting of a first positive lens and a first negative lens,
The objective optical system according to any one of Additional Items 1 to 3, characterized by satisfying the following conditional expression (11).
−1.6 <fG1Cp / fG1Cn <−0.4 (11)
fG1Cp is the focal length of the first positive lens,
fG1Cn is the focal length of the first negative lens,
It is.

(Appendix 12)
Has a brightness aperture,
The objective optical system according to any one of Appendices 1 to 3, wherein the following conditional expression (12) is satisfied.
0.3 <fGF / fGR <0.75 (12)
here,
fGF is the focal length of the front group at long distance object point focusing,
fGR is the focal length of the rear group at long distance object point focusing,
The front group is a lens group composed of all lens groups located on the object side of the aperture stop,
The rear group is a lens group composed of all lens groups located on the image side of the aperture stop,
It is.

(Appendix 13)
The objective optical system according to any one of Additional Items 1 to 3, characterized by satisfying the following conditional expression (13).
2.5 <Fno <5 (13)
here,
Fno is the f-number when focusing on a far distance object point,
It is.

  The present invention is useful for an objective optical system which is not susceptible to various errors, is bright, and various aberrations are well corrected.

G1 first lens group G2 second lens group G3 third lens group G4 fourth lens group L1 to L10 lens CL1, CL2, CL3 cemented lens S brightness stop F1, F2, F3 parallel plate I image plane (imaging surface)

Claims (3)

From the object side,
A first lens group of negative refractive power which is always stationary ,
A second lens unit of positive refractive power,
A third lens unit of negative refractive power,
A fourth lens unit having a positive refractive power which is stationary at all times, consists,
At the time of focusing from a far distance object point to a near distance object point, the second lens unit moves to the object side, and the third lens unit moves to the image side,
An objective optical system characterized by satisfying the following conditional expressions (1), (5) and (7).
2 <fG2 / f <8 (1)
0.2 <(t34f−t34n) / f <0.5 (5)
−8 <fG1 / f <−2 (7)
here,
fG2 is a focal length of the second lens group,
f is the focal length of the whole objective optical system when focusing on a far distance object point,
t34f is a distance between the third lens unit and the fourth lens unit when focusing on a far distance object point;
t34 n is a distance between the third lens unit and the fourth lens unit at the time of focusing on a near object point;
fG1 is a focal length of the first lens group,
It is. The objective optical system according to claim 1, wherein the following conditional expression (2) is satisfied.
-8 <fG3 / f <-2 (2)
here,
fG3 is a focal length of the third lens group,
f is the focal length of the whole objective optical system when focusing on a far distance object point,
It is. The objective optical system according to claim 1, wherein the following conditional expression (3) is satisfied.
0.5 <(t12f−t12n) / (t34f−t34n) <4 (3)
here,
t12f is a distance between the first lens unit and the second lens unit when focusing on a far distance object point;
t12 n is the distance between the first lens group and the second lens group at the time of focusing on a near object point;
t34f is a distance between the third lens unit and the fourth lens unit when focusing on a far distance object point;
t34 n is a distance between the third lens unit and the fourth lens unit at the time of focusing on a near object point;
It is.

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